v20250220-0017-plpgsql-implementation-for-LET-statement.patch

From 19a1be46e9e82680accbb6eb919048be30c399e6 Mon Sep 17 00:00:00 2001
From: "okbob@github.com" <okbob@github.com>
Date: Sat, 20 Jan 2024 08:56:17 +0100
Subject: [PATCH 17/21] plpgsql implementation for LET statement

PLpgSQL allows to call expression executor directly (for simple expression).
This possibility strongly reduces overhead related to execution. Reimplementation
of LET statement (inside PLpgSQL) allows to use this possibility, and strongly
increase performance:

CREATE VARIABLE svar int;

DO $$
BEGIN
  FOR i IN 1..10000
  LOOP
    LET svar = i;
  END LOOP;
END;
$$;

From 120ms to 8ms (with assertions) (this is best case, but without it, the LET statement
can be bottle neck).

An alternative can be reimplementation of LET statement inside expression
executor, and then SQL LET command can be executed in simple expression
execution, but this increase an complexity of executor, but still the
benefits is only inside plpgsql, so it is better to this optimization
inside plpgsql.
---
 src/backend/commands/session_variable.c       | 16 ++++++
 src/backend/executor/spi.c                    |  3 +
 src/backend/parser/analyze.c                  | 12 ++++
 src/backend/parser/gram.y                     |  8 +++
 src/backend/parser/parser.c                   |  1 +
 src/include/commands/session_variable.h       |  1 +
 src/include/nodes/parsenodes.h                |  2 +
 src/include/parser/parser.h                   |  4 ++
 src/pl/plpgsql/src/pl_exec.c                  | 56 +++++++++++++++++++
 src/pl/plpgsql/src/pl_funcs.c                 | 21 +++++++
 src/pl/plpgsql/src/pl_gram.y                  | 29 +++++++++-
 src/pl/plpgsql/src/pl_reserved_kwlist.h       |  1 +
 src/pl/plpgsql/src/pl_unreserved_kwlist.h     |  1 +
 src/pl/plpgsql/src/plpgsql.h                  | 12 ++++
 .../regress/expected/session_variables.out    |  6 +-
 src/tools/pgindent/typedefs.list              |  1 +
 16 files changed, 169 insertions(+), 5 deletions(-)

diff --git a/src/backend/commands/session_variable.c b/src/backend/commands/session_variable.c
index bb6abb792be..b08c1b47d38 100644
--- a/src/backend/commands/session_variable.c
+++ b/src/backend/commands/session_variable.c
@@ -948,6 +948,22 @@ SetSessionVariable(Oid varid, Datum value, bool isNull)
 	set_session_variable(svar, value, isNull);
 }
 
+/*
+ * Wrapper around SetSessionVariable with permission checks.
+ */
+void
+SetSessionVariableWithSecurityCheck(Oid varid, Datum value, bool isNull)
+{
+	AclResult	aclresult;
+
+	/* is the caller allowed to update the session variable? */
+	aclresult = object_aclcheck(VariableRelationId, varid, GetUserId(), ACL_UPDATE);
+	if (aclresult != ACLCHECK_OK)
+		aclcheck_error(aclresult, OBJECT_VARIABLE, get_session_variable_name(varid));
+
+	SetSessionVariable(varid, value, isNull);
+}
+
 /*
  * Returns a copy of the value stored in a variable.
  */
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 3288396def3..e3c4d259d53 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -3008,6 +3008,9 @@ _SPI_error_callback(void *arg)
 			case RAW_PARSE_PLPGSQL_ASSIGN3:
 				errcontext("PL/pgSQL assignment \"%s\"", query);
 				break;
+			case RAW_PARSE_PLPGSQL_LET:
+				errcontext("LET statement \"%s\"", query);
+				break;
 			default:
 				errcontext("SQL statement \"%s\"", query);
 				break;
diff --git a/src/backend/parser/analyze.c b/src/backend/parser/analyze.c
index 46a22ff5c94..2de0d23c8e3 100644
--- a/src/backend/parser/analyze.c
+++ b/src/backend/parser/analyze.c
@@ -2012,6 +2012,18 @@ transformLetStmt(ParseState *pstate, LetStmt *stmt)
 
 	stmt->query = (Node *) query;
 
+	/*
+	 * Inside PL/pgSQL we don't want to execute LET statement as utility
+	 * command, because it disallow to execute expression as simple
+	 * expression. So for PL/pgSQL we have extra path, and we return SELECT.
+	 * Then it can be executed by exec_eval_expr. Result is dirrectly assigned
+	 * to target session variable inside PL/pgSQL LET statement handler. This
+	 * is extra code, extra path, but possibility to get faster execution is
+	 * too attractive.
+	 */
+	if (stmt->plpgsql_mode)
+		return query;
+
 	/* represent the command as a utility Query */
 	result = makeNode(Query);
 	result->commandType = CMD_UTILITY;
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 53284c0289c..1c96bbf0819 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -824,6 +824,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %token		MODE_PLPGSQL_ASSIGN1
 %token		MODE_PLPGSQL_ASSIGN2
 %token		MODE_PLPGSQL_ASSIGN3
+%token		MODE_PLPGSQL_LET
 
 
 /* Precedence: lowest to highest */
@@ -955,6 +956,13 @@ parse_toplevel:
 				pg_yyget_extra(yyscanner)->parsetree =
 					list_make1(makeRawStmt((Node *) n, @2));
 			}
+			| MODE_PLPGSQL_LET LetStmt
+			{
+				LetStmt *n = (LetStmt *) $2;
+				n->plpgsql_mode = true;
+				pg_yyget_extra(yyscanner)->parsetree =
+					list_make1(makeRawStmt((Node *) n, 0));
+			}
 		;
 
 /*
diff --git a/src/backend/parser/parser.c b/src/backend/parser/parser.c
index 33a040506b4..887212aefde 100644
--- a/src/backend/parser/parser.c
+++ b/src/backend/parser/parser.c
@@ -62,6 +62,7 @@ raw_parser(const char *str, RawParseMode mode)
 			[RAW_PARSE_PLPGSQL_ASSIGN1] = MODE_PLPGSQL_ASSIGN1,
 			[RAW_PARSE_PLPGSQL_ASSIGN2] = MODE_PLPGSQL_ASSIGN2,
 			[RAW_PARSE_PLPGSQL_ASSIGN3] = MODE_PLPGSQL_ASSIGN3,
+			[RAW_PARSE_PLPGSQL_LET] = MODE_PLPGSQL_LET,
 		};
 
 		yyextra.have_lookahead = true;
diff --git a/src/include/commands/session_variable.h b/src/include/commands/session_variable.h
index f66b6c0b864..b89728ead5a 100644
--- a/src/include/commands/session_variable.h
+++ b/src/include/commands/session_variable.h
@@ -28,6 +28,7 @@ extern void AtEOSubXact_SessionVariables(bool isCommit, SubTransactionId mySubid
 										 SubTransactionId parentSubid);
 
 extern void SetSessionVariable(Oid varid, Datum value, bool isNull);
+extern void SetSessionVariableWithSecurityCheck(Oid varid, Datum value, bool isNull);
 extern Datum GetSessionVariable(Oid varid, bool *isNull);
 extern Datum GetSessionVariableWithTypeid(Oid varid, bool *isNull, Oid *typid);
 
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 785354ab52c..8d0d7cf6ec9 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2168,6 +2168,8 @@ typedef struct LetStmt
 	NodeTag		type;
 	List	   *target;			/* target variable */
 	Node	   *query;			/* source expression */
+	bool		plpgsql_mode;	/* true, when command will be executed
+								 * (parsed) by plpgsql runtime */
 	ParseLoc	location;
 } LetStmt;
 
diff --git a/src/include/parser/parser.h b/src/include/parser/parser.h
index 350196cd641..c8d5e8dbf92 100644
--- a/src/include/parser/parser.h
+++ b/src/include/parser/parser.h
@@ -33,6 +33,9 @@
  * RAW_PARSE_PLPGSQL_ASSIGNn: parse a PL/pgSQL assignment statement,
  * and return a one-element List containing a RawStmt node.  "n"
  * gives the number of dotted names comprising the target ColumnRef.
+ *
+ * RAW_PARSE_PLPGSQL_LET: parse a LET statement, and return a
+ * one-element List containing a RawStmt node.
  */
 typedef enum
 {
@@ -42,6 +45,7 @@ typedef enum
 	RAW_PARSE_PLPGSQL_ASSIGN1,
 	RAW_PARSE_PLPGSQL_ASSIGN2,
 	RAW_PARSE_PLPGSQL_ASSIGN3,
+	RAW_PARSE_PLPGSQL_LET,
 } RawParseMode;
 
 /* Values for the backslash_quote GUC */
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index d4377ceecbf..05eb4ac4f32 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -22,6 +22,7 @@
 #include "access/tupconvert.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_type.h"
+#include "commands/session_variable.h"
 #include "executor/execExpr.h"
 #include "executor/spi.h"
 #include "executor/tstoreReceiver.h"
@@ -333,6 +334,8 @@ static int	exec_stmt_commit(PLpgSQL_execstate *estate,
 							 PLpgSQL_stmt_commit *stmt);
 static int	exec_stmt_rollback(PLpgSQL_execstate *estate,
 							   PLpgSQL_stmt_rollback *stmt);
+static int	exec_stmt_let(PLpgSQL_execstate *estate,
+						  PLpgSQL_stmt_let *let);
 
 static void plpgsql_estate_setup(PLpgSQL_execstate *estate,
 								 PLpgSQL_function *func,
@@ -2132,6 +2135,10 @@ exec_stmts(PLpgSQL_execstate *estate, List *stmts)
 				rc = exec_stmt_rollback(estate, (PLpgSQL_stmt_rollback *) stmt);
 				break;
 
+			case PLPGSQL_STMT_LET:
+				rc = exec_stmt_let(estate, (PLpgSQL_stmt_let *) stmt);
+				break;
+
 			default:
 				/* point err_stmt to parent, since this one seems corrupt */
 				estate->err_stmt = save_estmt;
@@ -5009,6 +5016,55 @@ exec_stmt_rollback(PLpgSQL_execstate *estate, PLpgSQL_stmt_rollback *stmt)
 	return PLPGSQL_RC_OK;
 }
 
+/* ----------
+ * exec_stmt_let			Evaluate an expression and
+ *					put the result into a session variable.
+ * ----------
+ */
+static int
+exec_stmt_let(PLpgSQL_execstate *estate, PLpgSQL_stmt_let *stmt)
+{
+	bool		isNull;
+	Oid			valtype;
+	int32		valtypmod;
+	Datum		value;
+	Oid			varid;
+
+	List	   *plansources;
+	CachedPlanSource *plansource;
+
+	value = exec_eval_expr(estate,
+						   stmt->expr,
+						   &isNull,
+						   &valtype,
+						   &valtypmod);
+
+	/*
+	 * Oid of target session variable is stored in Query structure. It is
+	 * safer to read this value directly from the plan than to hold this value
+	 * in the plpgsql context, because it's not necessary to handle
+	 * invalidation of the cached value. Next operations are read only without
+	 * any allocations, so we can expect that retrieving varid from Query
+	 * should be fast.
+	 */
+	plansources = SPI_plan_get_plan_sources(stmt->expr->plan);
+	if (list_length(plansources) != 1)
+		elog(ERROR, "unexpected length of plansources of query for LET statement");
+
+	plansource = (CachedPlanSource *) linitial(plansources);
+	if (list_length(plansource->query_list) != 1)
+		elog(ERROR, "unexpected length of plansource of query for LET statement");
+
+	varid = linitial_node(Query, plansource->query_list)->resultVariable;
+	if (!OidIsValid(varid))
+		elog(ERROR, "oid of target session variable is not valid");
+
+	SetSessionVariableWithSecurityCheck(varid, value, isNull);
+	exec_eval_cleanup(estate);
+
+	return PLPGSQL_RC_OK;
+}
+
 /* ----------
  * exec_assign_expr			Put an expression's result into a variable.
  * ----------
diff --git a/src/pl/plpgsql/src/pl_funcs.c b/src/pl/plpgsql/src/pl_funcs.c
index 6b5394fc5fa..242b57f76de 100644
--- a/src/pl/plpgsql/src/pl_funcs.c
+++ b/src/pl/plpgsql/src/pl_funcs.c
@@ -288,6 +288,8 @@ plpgsql_stmt_typename(PLpgSQL_stmt *stmt)
 			return "COMMIT";
 		case PLPGSQL_STMT_ROLLBACK:
 			return "ROLLBACK";
+		case PLPGSQL_STMT_LET:
+			return "LET";
 	}
 
 	return "unknown";
@@ -585,6 +587,13 @@ plpgsql_statement_tree_walker_impl(PLpgSQL_stmt *stmt,
 				E_WALK(cstmt->expr);
 				break;
 			}
+		case PLPGSQL_STMT_LET:
+			{
+				PLpgSQL_stmt_let *lstmt = (PLpgSQL_stmt_let *) stmt;
+
+				E_WALK(lstmt->expr);
+				break;
+			}
 		case PLPGSQL_STMT_COMMIT:
 		case PLPGSQL_STMT_ROLLBACK:
 			{
@@ -806,6 +815,7 @@ static void dump_perform(PLpgSQL_stmt_perform *stmt);
 static void dump_call(PLpgSQL_stmt_call *stmt);
 static void dump_commit(PLpgSQL_stmt_commit *stmt);
 static void dump_rollback(PLpgSQL_stmt_rollback *stmt);
+static void dump_let(PLpgSQL_stmt_let *stmt);
 static void dump_expr(PLpgSQL_expr *expr);
 
 
@@ -905,6 +915,9 @@ dump_stmt(PLpgSQL_stmt *stmt)
 		case PLPGSQL_STMT_ROLLBACK:
 			dump_rollback((PLpgSQL_stmt_rollback *) stmt);
 			break;
+		case PLPGSQL_STMT_LET:
+			dump_let((PLpgSQL_stmt_let *) stmt);
+			break;
 		default:
 			elog(ERROR, "unrecognized cmd_type: %d", stmt->cmd_type);
 			break;
@@ -1581,6 +1594,14 @@ dump_getdiag(PLpgSQL_stmt_getdiag *stmt)
 	printf("\n");
 }
 
+static void
+dump_let(PLpgSQL_stmt_let *stmt)
+{
+	dump_ind();
+	dump_expr(stmt->expr);
+	printf("\n");
+}
+
 static void
 dump_expr(PLpgSQL_expr *expr)
 {
diff --git a/src/pl/plpgsql/src/pl_gram.y b/src/pl/plpgsql/src/pl_gram.y
index 8048e040f81..980ebb56a45 100644
--- a/src/pl/plpgsql/src/pl_gram.y
+++ b/src/pl/plpgsql/src/pl_gram.y
@@ -210,7 +210,7 @@ static	void			check_raise_parameters(PLpgSQL_stmt_raise *stmt);
 %type <stmt>	stmt_return stmt_raise stmt_assert stmt_execsql
 %type <stmt>	stmt_dynexecute stmt_for stmt_perform stmt_call stmt_getdiag
 %type <stmt>	stmt_open stmt_fetch stmt_move stmt_close stmt_null
-%type <stmt>	stmt_commit stmt_rollback
+%type <stmt>	stmt_commit stmt_rollback stmt_let
 %type <stmt>	stmt_case stmt_foreach_a
 
 %type <list>	proc_exceptions
@@ -317,6 +317,7 @@ static	void			check_raise_parameters(PLpgSQL_stmt_raise *stmt);
 %token <keyword>	K_INTO
 %token <keyword>	K_IS
 %token <keyword>	K_LAST
+%token <keyword>	K_LET
 %token <keyword>	K_LOG
 %token <keyword>	K_LOOP
 %token <keyword>	K_MERGE
@@ -891,6 +892,8 @@ proc_stmt		: pl_block ';'
 						{ $$ = $1; }
 				| stmt_rollback
 						{ $$ = $1; }
+				| stmt_let
+						{ $$ = $1; }
 				;
 
 stmt_perform	: K_PERFORM
@@ -1010,6 +1013,30 @@ stmt_assign		: T_DATUM
 					}
 				;
 
+stmt_let		: K_LET
+					{
+						PLpgSQL_stmt_let *new;
+						RawParseMode pmode;
+
+						pmode = RAW_PARSE_PLPGSQL_LET;
+
+						new = palloc0(sizeof(PLpgSQL_stmt_let));
+						new->cmd_type = PLPGSQL_STMT_LET;
+						new->lineno   = plpgsql_location_to_lineno(@1, yyscanner);
+						new->stmtid = ++plpgsql_curr_compile->nstatements;
+
+						/* push back the head name to include it in the stmt */
+						plpgsql_push_back_token(K_LET, &yylval, &yylloc, yyscanner);
+						new->expr = read_sql_construct(';', 0, 0, ";",
+													   pmode,
+													   false, true,
+													   NULL, NULL,
+													   &yylval, &yylloc, yyscanner);
+
+						$$ = (PLpgSQL_stmt *)new;
+					}
+				;
+
 stmt_getdiag	: K_GET getdiag_area_opt K_DIAGNOSTICS getdiag_list ';'
 					{
 						PLpgSQL_stmt_getdiag *new;
diff --git a/src/pl/plpgsql/src/pl_reserved_kwlist.h b/src/pl/plpgsql/src/pl_reserved_kwlist.h
index ce7b0c9d331..e266bd1cc46 100644
--- a/src/pl/plpgsql/src/pl_reserved_kwlist.h
+++ b/src/pl/plpgsql/src/pl_reserved_kwlist.h
@@ -40,6 +40,7 @@ PG_KEYWORD("from", K_FROM)
 PG_KEYWORD("if", K_IF)
 PG_KEYWORD("in", K_IN)
 PG_KEYWORD("into", K_INTO)
+PG_KEYWORD("let", K_LET)
 PG_KEYWORD("loop", K_LOOP)
 PG_KEYWORD("not", K_NOT)
 PG_KEYWORD("null", K_NULL)
diff --git a/src/pl/plpgsql/src/pl_unreserved_kwlist.h b/src/pl/plpgsql/src/pl_unreserved_kwlist.h
index 98f99ec470c..ed14fd86c31 100644
--- a/src/pl/plpgsql/src/pl_unreserved_kwlist.h
+++ b/src/pl/plpgsql/src/pl_unreserved_kwlist.h
@@ -69,6 +69,7 @@ PG_KEYWORD("info", K_INFO)
 PG_KEYWORD("insert", K_INSERT)
 PG_KEYWORD("is", K_IS)
 PG_KEYWORD("last", K_LAST)
+PG_KEYWORD("let", K_LET)
 PG_KEYWORD("log", K_LOG)
 PG_KEYWORD("merge", K_MERGE)
 PG_KEYWORD("message", K_MESSAGE)
diff --git a/src/pl/plpgsql/src/plpgsql.h b/src/pl/plpgsql/src/plpgsql.h
index d73996e09c0..c7538612f1a 100644
--- a/src/pl/plpgsql/src/plpgsql.h
+++ b/src/pl/plpgsql/src/plpgsql.h
@@ -128,6 +128,7 @@ typedef enum PLpgSQL_stmt_type
 	PLPGSQL_STMT_CALL,
 	PLPGSQL_STMT_COMMIT,
 	PLPGSQL_STMT_ROLLBACK,
+	PLPGSQL_STMT_LET,
 } PLpgSQL_stmt_type;
 
 /*
@@ -542,6 +543,17 @@ typedef struct PLpgSQL_stmt_assign
 	PLpgSQL_expr *expr;
 } PLpgSQL_stmt_assign;
 
+/*
+ * Let statement
+ */
+typedef struct PLpgSQL_stmt_let
+{
+	PLpgSQL_stmt_type cmd_type;
+	int			lineno;
+	unsigned int stmtid;
+	PLpgSQL_expr *expr;
+} PLpgSQL_stmt_let;
+
 /*
  * PERFORM statement
  */
diff --git a/src/test/regress/expected/session_variables.out b/src/test/regress/expected/session_variables.out
index d0658116683..c25d804f8c1 100644
--- a/src/test/regress/expected/session_variables.out
+++ b/src/test/regress/expected/session_variables.out
@@ -1129,8 +1129,7 @@ ERROR:  permission denied for session variable var1
 CONTEXT:  PL/pgSQL function inline_code_block line 1 at RAISE
 DO $$ BEGIN LET var1.a = var1.a + 10; END $$;
 ERROR:  permission denied for session variable var1
-CONTEXT:  SQL statement "LET var1.a = var1.a + 10"
-PL/pgSQL function inline_code_block line 1 at SQL statement
+CONTEXT:  PL/pgSQL function inline_code_block line 1 at LET
 SET ROLE TO DEFAULT;
 GRANT SELECT ON VARIABLE var1 TO regress_var_test_role;
 SET ROLE TO regress_var_test_role;
@@ -1174,8 +1173,7 @@ ERROR:  permission denied for session variable var1
 CONTEXT:  PL/pgSQL function inline_code_block line 1 at RAISE
 DO $$ BEGIN LET var1.a = var1.a + 10; END $$;
 ERROR:  permission denied for session variable var1
-CONTEXT:  SQL statement "LET var1.a = var1.a + 10"
-PL/pgSQL function inline_code_block line 1 at SQL statement
+CONTEXT:  PL/pgSQL function inline_code_block line 1 at LET
 SET ROLE TO DEFAULT;
 DROP VARIABLE var1;
 DROP TABLE vartesttab;
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 794cd7b7619..7ff39e9a0d2 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -1907,6 +1907,7 @@ PLpgSQL_stmt_forq
 PLpgSQL_stmt_fors
 PLpgSQL_stmt_getdiag
 PLpgSQL_stmt_if
+PLpgSQL_stmt_let
 PLpgSQL_stmt_loop
 PLpgSQL_stmt_open
 PLpgSQL_stmt_perform
-- 
2.48.1